The genotype of hepatitis C virus (HCV) and the amount of HCV RNA are often used to predict the efficacy of interferon (IFN) therapy on chronic hepatitis C. In addition to these factors, there may be several factors related to the host. Therefore, the authors undertook a retrospective study in which physical findings and laboratory data before therapy were evaluated by multiple logistic analysis. Two-hundred and five cases with chronic hepatitis C treated with interferon were analyzed in this study. Sustained virological response was observed with 68 cases. Multiple logistic analysis was performed with 29 explanatory variables including HCV genotype, HCV RNA, IFN types, and total dose, along with gender, age, alcohol consumption, body mass index (BMI), histological findings of liver biopsy, platelet counts, and laboratory data of serum enzymes. Analysis on the factors that correlated well with therapeutic efficacy revealed that genotype 2a, 2b showed higher therapeutic responses than genotype 1b with reference to HCV genotypes, and higher IFN dose or lower HCV RNA levels gave better results. With reference to host factors, higher total protein level, lower levels of BMI, total bilirubin, and aspartate aminotransferase were highly correlated with therapeutic efficacy. HCV genotypes and HCV RNA levels have been already identified as prognostic factors. However, the high correlation values of BMI and the total protein level are new findings. It is suggested that probability estimation of therapeutic effects using the logistic regression equation may be a good tool for predicting therapeutic efficacy of IFN therapy on individual cases.